Ultrasonic flowmeter

ABSTRACT

An ultrasonic flowmeter, wherein upstream and downstream receive signals are A/D-converted and thereby the discrete values thereof are determined: a cross-correlation between the upstream and downstream receive signals is determined: the result of calculation by correlation processing means is Hilbert-transformed; a phase relationship is determined by phase calculating means from the results of calculation by the correlation processing means and calculation by Hilbert transform; and a time difference in calculated from the phase calculation result provided by the phase calculating means through maximum value decision means.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an ultrasonic flowmeter forA/D-converting an ultrasonic signals calculating a discrete correlationfactor and determining a time difference equivalent to a flow velocity.

[0003] 2. Description of the Prior Art

[0004] The method of determining a flow velocity, i.e. a timedifference, using a prior art ultrasonic flowmeter, according to acorrelation factor between ultrasonic signals passing through a fluidhas the advantage of being resistant to disturbance, such an noise. Inorder to determine the correlation factor between ultrasonic signals,the signals must be A/D-converted and then correlated. The correlationfactor, therefore, results in a discrete value, Thus, In order toevaluate a precise time difference equivalent to a flow velocity fromthe resulting discrete values, the discrete data must be interpolated asshown in FIG. 1. Then, the time at which the correlation factor is at amaximum must be calculated from the interpolated discrete data.

[0005] In order to evaluate the maximum value of a correlation factor,it is common to use higher-order curvilinear interpolation. Thisapproach requires a large amount of data, however. To be able to acquirea large amount of data, it is necessary to use extremely fast,high-resolution A/D converters and make complex higher-order curvilinearinterpolation calculations. Conversely, curvilinear interpolation usinga small amount of data will result in a larger calculation error. Suchhigh-speed high-resolution A/D converters are extremely costly. Inaddition, higher-order curvilinear interpolation calculations tend tobecome complex.

SUMMARY OF THE INVENTION

[0006] The present invention has been made in the light of theabove-noted problems. An object of the invention is, therefore, toprovide an ultrasonic flowmeter that implements the method ofcalculating a time difference with improved accuracy and at higherspeeds using relatively simple signal processing means and therebydetermining the maximum value of a correlation factor.

[0007] In order to attain the above-noted object, the ultrasonicflowmeter in a first aspect of the present invention recited in claim 1is defined as one for determining a flow rate according to ultrasonicsignals passing through a fluid, comprising;

[0008] calculating means for A/D-converting and thereby discretizing anupstream receive signal and a downstream receive signal;

[0009] correlation processing means for determining a cross-correlationbetween the upstream receive signal and downstream receive signal;

[0010] calculating means for Hilbert-transforming the result ofcalculation made by the correlation processing means;

[0011] phase calculating means for determining a phase relationship fromthe results of calculation provided by the correlation processing meansand Hilbert transform means; and

[0012] first calculating means for calculating a time differenceaccording to the result of phase relationship calculation made by thephase calculating means.

[0013] Therefore, according to the first aspect of the present inventionrecited in claim 1, upstream and downstream receive signals arediscretized without having to make complex higher-order curvilinearinterpolation calculations; a correlation factor is determined usingcorrelation processing means; the result of Hilbert calculation providedby Hilbert transform means in obtained; and the maximum value of thecorrelation factor is determined by making a phase calculation from thecorrelation factor and the result of the Hilbert calculation.

[0014] In a second aspect of the present invention as recited in claim2, the ultrasonic flowmeter of the present invention is defined an onefor determining a flow rate according to ultrasonic signals passingthrough a fluid, comprising:

[0015] calculating means for A/D-converting and thereby discretizing anupstream receive signal and a downstream receive signal;

[0016] correlation processing means for determining a cross-correlationbetween the upstream receive signal and the downstream receive signal;

[0017] calculating means for Hilbert-transforming the result ofcalculation made by the correlation processing means;

[0018] phase calculating means for determining a phase relationship fromthe result of calculation made by the correlation processing means andthe result of calculation provided by the Hilbert transform means;

[0019] first calculating means for determining a time difference fromthe result of phase relationship calculation made by the phasecalculating means;

[0020] calculating means for Hilbert-transforming the upstream receivesignal and the downstream receive signal;

[0021] calculating means for determining an envelope from the upstreamreceive signal, the downstream receive signal, and the result ofcalculation provided by the Hilbert transform means;

[0022] second calculating means for determining a time difference fromthe envelope; and

[0023] a function for determining a flow velocity dependent timedifference from the time difference determined by the second calculatingmeans and the time difference determined by the first calculating means.

[0024] Therefore, according to the second aspect of the presentinvention recited in claim 2, upstream and downstream receive signalsare Hilbert-transformed; an envelope is determined from the result ofHilbert transform based calculation of the upstream and downstreamreceive signals; an approximate time difference ri calculated from theenvelope: an accurate time difference, which is the maximum value of theapproximate time difference, in then determined from the result of phasecalculation; thus making it possible to identify a true time differencewhen the correlation factor proves to be multiple-peaked.

[0025] In a third aspect of the present invention as recited in claim 3,the ultrasonic flowmeter of the present invention is defined as one fordetermining a flow rate according to ultrasonic signals passing througha fluid, comprising:

[0026] calculating means for A/D-converting and thereby discretizing anupstream receive signal and a downstream receive signal;

[0027] correlation processing means for determining a cross-correlationbetween the upstream receive signal and the downstream receive signal;

[0028] calculating means for Hilbert-transforming the result ofcalculation made by the correlation processing means;

[0029] phase calculating means for determining a phase relationship fromthe results of calculation provided by the correlation processing meansand Hilbert transform means;

[0030] first calculating means for determining a time differenceaccording to the result of phase relationship calculation made by thephase calculating means;

[0031] third calculating means for determining an envelope from thegeometric mean of the results of calculation provided by the correlationprocessing means and Hilbert transform means, and then calculating atime difference from the maximum value of the envelope: and

[0032] a function for determining a flow velocity dependent timedifference from the time difference determined by the third calculatingmeans and the time difference determined by the first calculating means.

[0033] Therefore, according to the third aspect of the present Inventionrecited in claim 3, an envelope is determined from the geometric mean ofa correlation factor obtained by the oorrelation processing means fromthe correlation between upstream and downstream receive signals and theresult of calculation provided by Hilbert transform means; anapproximate time difference is calculated from the maximum value of theenvelopes an accurate time difference, which is the maximum value of theapproximate time difference, is then determined from the result of phasecalculation; thus making it possible to identify the true timedifference when the correlation factor proves to be multiple-peaked.

[0034] In a fourth aspect of the present invention as recited in claim4. the ultrasonic flowmeter of the present invention is defined as onefor determining a flow rate according to ultrasonic signals passingthrough a fluid, comprising:

[0035] calculating means for A/D-converting and thereby discretizing anupstream receive signal and a downstream receive signal;

[0036] correlation processing means for determining a cross-correlationbetween the upstream receive signal and the downstream receive signal;

[0037] calculating means for Hilbert-transforming the result ofcalculation made by the correlation processing means;

[0038] phase calculating means for determining a phase relationship fromthe results of calculation provided by the correlation processing meansand Hilbert transform means;

[0039] first calculating means for determining a time difference fromthe result of phase relationship calculation made by the phasecalculating means;

[0040] fourth calculating means for determining the auto-correlationsbetween upstream transmit and receive signals and between downstreamtransmit and receive signals respectively and thereby calculating a timedifference; and

[0041] a function for determining a flow velocity dependent timedifference according to the time difference determined by the fourthcalculating means and the time difference determined by the firstcalculating means.

[0042] Therefore, according to the fourth aspect of the presentinvention recited in claim 4, the auto-correlations between upstreamtransmit and receive signals and between downstream transmit and receivesignals are determined respectively; an approximate time difference iscalculated from the auto-correlations; and the accurate time difference,which is the maximum value of the approximate time difference, is thendetermined from the result of phase calculation; thus making it possibleto identify the true time difference when the correlation factor provesto be multi-peaked.

BRIEF DESCRIPTION OF THE DRAWINGS

[0043]FIG. 1 in a conventional graph used to determine a precise timedifference equivalent to a flow velocity.

[0044]FIG. 2 is a flow chart representing a basic system for processingultrasonic signals according to the present invention.

[0045]FIG. 3 is a waveform chart showing the result of A/D-convertingand then the graphing of an ultrasonic receive signal.

[0046]FIG. 4 Is a waveform chart obtained by graphing the results ofcalculating the receive signals shown in FIG. 3 in each processing step,wherein:

[0047]FIG. 4a is a waveform chart showing the result of calculating across-correlation between the receive signals of FIG. 3, the Hilberttransform of the calculation result, and a phase relationship thusdetermined; and

[0048]FIG. 4b is a close-up of the waveform chart shown in FIG. 4a.

[0049]FIG. 5 is a graphical waveform chart showing the result ofcorrelation processing, wherein:

[0050]FIG. 5a is a graphical waveform chart showing a regular result ofcorrelation processing; and

[0051]FIG. 5b is a graphical waveform chart showing a multiple-peakedresult of correlation processing.

[0052] PIG. 6 is a flow chart representing a basic system for processingultrasonic signals according to the present invention, wherein acorrelation factor which is the result of correlation processing prove.to be multiple-peaked.

[0053]FIG. 7 is a simplified graphical representation of an algorithmfor calculating the arrival time of an ultrasonic receive signalaccording to the present invention, wherein a correlation factor whichis the result of correlation processing proves to be multiple-peaked.

[0054]FIG. 8 is a waveform chart obtained by graphing an envelopeactually calculated according to an ultrasonic receive signal.

[0055]FIG. 9 Is a waveform chart obtained by graphing an envelopedetermined by finding the geometric mean of the results of calculating across-correlation between upstream and downstream receive signals andHilbert transforming the cross-correlation.

[0056]FIG. 10 is a flow charts representing a system for processingultrasonic signals according to the present invention, wherein acorrelation factor which is the result of correlation processing provesto be multiple-peaked.

[0057]FIG. 11 in a flow chart representing a case where the two Hilberttransform units in the system of FIG. 6 are reduced to one.

[0058]FIG. 12 in a flow chart representing a case where a plurality ofcorrelation processing means are used in the system of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0059] Modes for carrying out the present invention are describedhereafter with reference to the accompanying drawings. FIG. 2schematically shows the basic configuration of an ultrasonic flowmeterwith regard to the processing of ultrasonic signals. In the figure,ultrasonic receive signals (one each on the upstream and downstreamsides) passing through a fluid are changed into discrete values with A/Dconverters (steps S1 and S2). From the result of conversion, across-correlation between the upstream receive signal (ultrasonic signalpropagating from the upstream side to the downstream side) anddownstream receive signal (ultrasonic signal propagating from thedownstream side to the upstream aide) is determined by correlationprocessing means (step S3).

[0060] Here, the value evaluated by the correlation processing means inassumed to be C(τ) (τ is a value equivalent to a time difference).

[0061] Also evaluated is H(τ) Which is the result of Hilbert transformcalculation provided by Hilbert transform means (Step 4). Then, a phaseis calculated by phase calculating means using C(τ) and H(τ) (step 5).From the result of phase calculation, the maximum value of a correlationfactor (time difference) is determined by maximum value decision meansfirst calculating means (step S6).

[0062] As a rule, a receive signal transmitted through a fluid anddetected in an ultrasonic flowmeter has such a waveform as shown in FIG.3. The waveform of FIG. 3 has been obtained by graphing the result ofA/D converting a receive signal detected after actually transmitting aburst wave through the wall of an BUS stainless-steel pipe into a fluid.in this graphical waveform chart, the receive signal is normalized withregard to strength and the horizontal axis represents time.

[0063] The fundamental frequency of the waveform (carrier frequency)equals the resonant frequency of a transmitting device, for example, apiezoelectric device serving as a transmitter, Any waveform obtained atthis point can be regarded an a sine wave. In addition, the shape of anenveloped wave depends on the resonance of a measuring system as awhole, for example, resonance due to multiple reflection within thethickness of a pipe wall. For burst waves, the shape depends on the timeinterval of the waves, noise that may occur in the measuring system, andso on.

[0064] Now, the result of correlation factor (correlation function)calculation made by the correlation processing means shown in FIG. 2 isapproximated to know what waveform the correlation function willprovide. As shown in FIG. 3, the receive signal can be assumed to be asine wave with a frequency dependent on the transmitting device. Alsolet's assume that the transmission frequency f0, the time differencebetween the upstream and downstream receive signals is δ, and the randomvariable uniformly distributed within the interval [0. 2π] is θ.

[0065] Then, it is possible to represent the upstream and downstreamreceive signals by equation 1, with reference to the time of thedownstream receive signal.

Upstream: sin (27πf0t+δ), Downstream: sin (27πf0t)  Equation 1

[0066] The cross-correlation function C(τ) is represented by equation 2below. $\begin{matrix}\begin{matrix}{{C(\tau)} = \quad {\int_{0}^{2\pi}{{\frac{1}{2\pi} \cdot \left\lbrack \left\lbrack {{{\sin \left( {{2\pi \quad {fot}} + \theta + \delta} \right)} \cdot {\sin \left\lbrack {2\pi \quad {{fo}\left( {t + \tau} \right)}} \right\rbrack}} + \theta} \right\rbrack \right\rbrack}{\theta}}}} \\{= \quad {\frac{1}{2} \cdot {\cos \left( {\delta - {2\pi \quad {fo}\quad \tau}} \right)}}}\end{matrix} & {{Equation}\quad 2}\end{matrix}$

[0067] As is evident from equation 2, the cross-correlation function ofa nine wave having frequency f0 has the same frequency as f0.

[0068] Now, as the result of Hilbert-transforming the cross-correlationfunction C(τ). equation 4 is derived from equation 2, instead ofequation 3 also derived therefrom.

cos (δ−2πf0τ)  Equation 3

H(τ)=sin {δ−2πf0τ)}  Equation 4

[0069] Now, regarding the phase calculating means, it is possible todefine a phase relationship P(τ) between C(τ) and H(τ) by equation 5.

P(τ)=tan⁻¹(H(τ)/C(τ))  Equation 5

[0070] Consequently, equation 6 holds true.

C(τ)/H(τ)=sin {−(δ−2πf₀τ)}/cos(δ−2πf0τ)=˜tan(δ−2πf0τ)  Equation 6

[0071] Hence, equation 7 can be obtained.

P(τ)=2f0τ−δ  Equation 7

[0072] Thus, the zero-cross point of the phase P(τ) is equivalent to themaximum value of the correlation function, which is equal to the timedifference and to the flow velocity as well. In practice, δ isdetermined from discrete time data. Since the phase relationshipequation is linear with respect to time τ, it is theoretically possibleto precisely determine the time difference by means of linearapproximation. This in still true even if actual data carriesfluctuations due to, for example, noise.

[0073] In FIG. 4, the result of processing respective ultrasonic receivesignals is graphed. A dotted line in FIG. 4a shows the result ofcalculating the cross-correlation function C(τ), a chain line shows theHilbert transform H(τ) of the cross-correlation function, and a chaindouble-dashed line shows the phase relationship between the twofunctions. Thus, it is evident from either FIG. 4a or FIG. 4b, which isa close-up of FIG. 4a, that δ can be easily determined from discretedata by means of linear approximation.

[0074] This result of signal processing recurs at an interval of 0 to 2πand the result of phase calculation provides a recurrent solution. Toallow the largest value to be determined, comparison among obtainedmaximum values must be made or different means for predicting themaximum value discussed later must be used concurrently.

[0075] Referring back to FIG. 2, it is possible to incorporate pluralityinto the correlation processing means. It is also possible to includeplural units serving as correlation processing means dedicated toHilbert transform, as shown in FIG. 12, thereby increasing the overallprocessing speed.

[0076] Another mode for carrying out the present invention will bedescribed now, wherein additional steps are included in the basicflowmeter configuration to cope with a case where a correlation factorobtained an the result of correlation processing is multiple-peaked. Inthe case of a clamp-on ultrasonic flowmeter in particular. thecorrelation factor may become multiple-peaked for such a reason that anultrasonic wave Causes multiple reflection due to thin-walled piping.FIG. 5 a shows a regular result of correlation processing, whereas FIG.5b illustrates a multiple-peaked result. If the heights of peaks areanalogous to each other, an shown in FIG. 5b, it is difficult todetermine which is the true peak, i.e., the true time difference.

[0077] Accordingly, processing based on the flowmeter configurationshown in FIG. 6 is applied to multiple-peaked signals. Added to theflowmeter configuration of FIG. 2 is means for identifying anapproximate time in order to determine what degree of time difference ismost plausible in a given case.

[0078] The system for detecting multiple-peaked signals is based on theprinciple that an envelope is determined by means of Hilbert transform,and then a time difference in the arrival of receive signals isdetermined from the envelope. The maximum value decision means used hereis the same as the one shown in FIG. 2. In this mode, however, themaximum value decision means will provide many solutions if the signalis multiple-peaked. In order to identify a correct solution, anothermeans is used and thereby an approximate time difference in the arrivalof ultrasonic receive signals is determined in such a way an describedin the following paragraphs. The algorithm for calculating the arrivaltime of these receive signals is shown in FIG. 7. The method ofidentification is described hereafter with reference to FIGS. 6 and 7.

[0079] Firstly, upstream and downstream ultrasonic receive signals arechanged into discrete values with A/D converters (steps S51 and S52).From the values, a cross-correlation between the upstream and downstreamultrasonic receive signals is determined by correlation processing means(step S53). The correlation factor thus determined isHilbert-transformed to obtain the result of Hilbert transformcalculation (step S54). Then, a phase relationship is calculated byphase calculating means using the correlation factor and the result ofHilbert transform calculation (step S55).

[0080] Next, the upstream and downstream ultrasonic receive signals aredigitized (A/D-converted), as shown in FIG. 6, by the additional meansincorporated in the flowmeter configuration of FIG. 2. From the discretevalues thus obtained, a Hilbert transform in determined (step S57).Then, from the receive signals and the result of Hilbert transform, anenvelope is calculated. The envelope in given by equation 8 below, wherethe ultrasonic receive signal is S and the Hilbert transform thereof isH.

{square root}{square root over (S²+H²)}  Equation 8

[0081]FIG. 8 shows the result of actual calculation based on a givenultrasonic receive signal. The figure indicates that an envelope iscalculated correctly according to equation 8. Zero-cross points aredetermined from the slopes of the envelope by interpolation and therebythe arrival time of the signal is determined. This process is carriedout by time measurement means (second calculating means) for bothupstream and downstream receive signals. A difference in the arrivaltime is used as an approximate time difference (step s58). On the otherhand, the maximum value decision means shown in FIG. 2 is used for timedifference options that provide plural maximum values. With the maximumvalue decision means, the true maximum value (time difference) indetermined from the result of phase calculation and the result ofcalculation made by the time measurement means (step S56).

[0082] In this mode, it is possible to use the Hilbert transformalgorithm of the basic flowmeter configuration as is. Another advantageof this process is that the algorithm is less likely to produce a largeerror due to such accidental noise as might be generated whenstraightforwardly determining threshold levels for a signal. This inbecause the algorithm determines an envelope by extrapolation andevaluates the signal using zero-cross points.

[0083] Yet another advantage in that the process only requires such adegree of time resolution as to enable any given peak to bediscriminated from an adjacent peak or peaks. As discussed in theprinciple of cross-correlation, the cross-correlation function sharesthe same frequency, i.e., f0=1 to a few megahertz, with the transmittingdevice. This means a time resolution of 0.1 to 1 μsec will suffice forone period of a signal. This value is readily attainable since a timedifference caused by flow velocity is, however small it may be, in theorder of several tow or hundred picoseconds.

[0084] Another point to note about this mode is that it is possible tomake shared use of only one Hilbert transform unit, instead of using twoseparately. one specific configuration example is shown in FIG. 12. Inthis configuration, the single Hilbert transform unit may first be usedto transform a receive signal during correlation processing. then, uponcompletion of the correlation processing, the unit may be used onceagain by means of a switching signal in order to perform another Hilberttransform for correlation processing.

[0085] Yet another point to note is that, in FIGS. 7 and 8, the methodof determining the arrival time of a signal from the result of envelopecalculation is by no means limitative. In an earlier discussionregarding FIG. 7, however, it is stated that the arrival time of asignal is determined simply by extrapolating an envelope. It should benoted here that the ultimate goal meant by this statement is todetermine an approximate time difference. Thus, there may be such analternative method that curvilinear approximation in the second or soorder in carried out using several plots of data. Yet anotheralternative method with regards to FIG. 9 may be performinginterpolation with a second-order curve and determining a differentialcoefficient, or finding an average using several plots of data thatexceed a given threshold.

[0086] The following system is available as another multiple-peakedsignal detection system (system for discriminating among multiple peaksof a signal) in this mode of carrying out the present invention. Thatis, the geometric mean of the results of calculating a cross-oorrelationbetween two signals and Hilbert-transforming the result of correlationcalculation is determined. These two values are those of sine and cosinefunctions, as indicated by equations 3 and 4. Finding the geometric meanthereof, therefore, results in the calculation of an envelope.Consequently, a value equivalent to an amplitude is determined, as shownin FIG. 9.

[0087] This means, the time determined to be concurrent with the maximumvalue of the amplitude corresponds to an approximate time difference(third calculating means). Then, from the approximate time differencethus determined and the result of calculation made by the phasecalculating means, the maximum value of a correlation factor isevaluated. More specifically, the method of determining the approximatetime difference discussed here and a method of determining a precisetime difference given by equation 7 may be combined so that a correctand precise time difference is evaluated from the result of correlationprocessing of a multiple-peaked signal.

[0088] The system discussed below is available as yet anothermultiple-peaked signal detection system (system for discriminating amongmultiple peaks of a signal). FIG. 10 shows the flow chart of signalprocessing performed by the multiple-peaked signal detection system. Thesystem employs the conventional method wherein an auto-correlation isused in order to determine an approximate time difference. In themultiple-peaked signal detection system, the approximate time differenceis determined by finding upstream and downstream time differences fromauto-correlations between transmit and receive signals. The systemitself carries out the same process as that performed by correlationprocessing means used by regular ultrasonic flowmeters.

[0089] Specifically, upstream and downstream ultrasonic receive signalare changed into discrete values with A/D converters, an shown in FIG.10 (steps S71 and S72). From the discrete values, a cross-correlationbetween the upstream and downstream receive signals is determined bycorrelation processing means (step S73). The correlation factor thusdetermined is then Hilbert-transformed to obtain the result ofHilbert-transformation calculation (step S74). A phase relationship isdetermined by phase calculating means using the correlation factor andthe result of Hilbert transform (step S75).

[0090] In a further step, correlation processing is carried out in anupstream correlation processing unit 40. Accordingly, upstream anddownstream ultrasonic receive signals are changed into discrete valueswith A/D converters (steps S91 and S92). From the discrete values, anauto-correlation between the upstream transmit and receive signals isdetermined by correlation processing means (step S93). Also in adownstream correlation processing unit 50, upstream and downstreamultrasonic receive signals are changed into discrete values with A/Dconverters (steps S94 and S95). Thus, an auto-correlation is determinedby phase correlation processing means (step S96).

[0091] In a still further step, an approximate time difference isdetermined by time difference calculating means (fourth calculatingmeans) using the auto-correlations determined by the correlationprocessing means as described above (step S97). From the correlationfactor determined by correlation processing means (step S3), the resultof phase calculation made by phase calculating means using the result ofHilbert transform calculation provided by Hilbert transform (step S75).and the result of time difference calculation made by the timedifference calculating means (step S97), a precise time difference,i.e., the maximum value of the correlation factor. is determined by themaximum value decision means (step S76).

[0092] Some algorithmic contrivance may be included in the above-notedunits for auto-correlation processing. For example, the units may bedesigned not to use waveforms or calculation results that are obviouslyabnormal, including cases when an evaluated time value or correlationfactor value is too small.

[0093] In another aspect of the present invention, algorithmiccontrivance may also be included in many other parts of the flowmeterconfiguration. For example, the flowmeter configuration may be designednot to use data values, which have deviated greatly from those of anexpected waveform or correlation factor due to accidental noise, in anyfurther processing step.

[0094] In yet another aspect of the present invention, across-correlation between receive signals is mentioned as an example. Itis also possible, however, to use an auto-correlation between a transmit(modulating) signal and a receive signal. This approach will make itpossible to determine a precise time value from the auto-correlation bymeans of interpolation using Hilbert transformation. The speed of soundcan also be evaluated by precisely measuring the propagation time of thesignal.

[0095] According to a first aspect of the present invention recited inclaim 1, it is possible to attain such a time resolution an to ensurehigh processing accuracy, by using a relatively simple flowmeterconfiguration. this can be implemented without having to make complexhigher-order curvilinear interpolation calculations, while at the sametime retaining the inherent feature of a correlation method that themethod is highly immune to accidental changes in a signal due todisturbance.

[0096] According to second to fourth aspects of the present inventionrecited in claims 2 to 4, it is possible to identify a true timedifference in a case where a given correlation factor proven to bemultiple-peaked.

What is claimed is:
 1. An ultrasonic flowmeter for determining a flowrate according to ultrasonic signals passing through a fluid,comprising: calculating means for A/D-converting and therebydiscretizing an upstream receive signal and a downstream receive signal;correlation processing means for determining a cross-correlation betweensaid upstream receive signal and said downstream receive signal;calculating meant for Hilbert-transforming the result of correlationprocessing provided by said correlation processing means; phasecalculating means for determining a phase relationship from the resultsof calculation provided by said correlation processing means and saidHilbert transform means; and first calculating means for calculating atime difference from the result of phase relationship calculationprovided by said phase calculating means.
 2. An ultrasonic flowmeter fordetermining a flow rate according to ultrasonic signals passing througha fluid, comprising: calculating means for A/D-converting and therebydiscretizing an upstream receive signal and a downstream receive signal;correlation processing means for determining a cross-correlation betweensaid upstream receive signal and said downstream receive signal;calculating means for Hilbert-transforming the result of correlationprocessing provided by said correlation processing means; phasecalculating means for determining a phase relationship from the resultsof calculation provided by said correlation processing means and saidHilbert transform means; first calculating means for determining a timedifference from the result of phase relationship calculation provided bysaid phase calculating means; calculating means for Hilbert-transformingsaid upstream receive signal and said downstream receive signal;calculating means for determining an envelope from said upstream receivesignal, said downstream receive signal and the result of calculationprovided by said Hilbert transform means; second calculating means fordetermining a time difference from said envelope; and a function fordetermining a flow velocity dependent time difference from timedifferences determined by said second calculating means and said firstcalculating means.
 3. An ultrasonic flowmeter for determining a flowrate according to ultrasonic signals passing through a fluid,comprising: calculating means for A/D-converting and therebydiscretizing an upstream receive signal and a downstream receive signal;correlation processing means for determining a cross-correlation betweensaid upstream receive signal and said downstream receive signal;calculating means for Hilbert-transforming the result of calculationprovided by said correlation processing means; phase calculating meansfor determining a phase relationship from the results of calculationprovided by said correlation processing means and said Hilbert transformmeans; first calculating means for determining a time difference fromthe result of phase relationship calculation provided by said phasecalculating means; third calculating means for determining an envelopefrom the geometric mean of the results of calculation provided by saidcorrelation processing means and said Hilbert transform means, and thencalculating a time difference from the maximum value of said envelope;and a function for determining a flow velocity dependent time differencefrom time differences determined by said third oalculating means andsaid first calculating means.
 4. An ultrasonic flowmeter for determininga flow rate according to ultrasonic signals passing through a fluid,comprising: calculating means for A/D-converting and therebydiscretizing an upstream receive signal and a downstream receive signal;correlation processing means for determining a cross-correlation betweensaid upstream receive signal and said downstream receive signal;calculating means for Hilbert-transforming the result of calculationprovided by said oorrelation processing means; phase calculating meansfor determining a phase relationship from the results of calculationprovided by said correlation processing means and said Hilbert transformmeans; first calculating means for determining a time difference fromthe result of phase relationship calculation provided by said phasecalculating means; fourth calculating means for determiningauto-correlations between upstream ultrasonic transmit and receivesignals and between downstream ultrasonic transmit and receive signals,respectively, and thereby calculating a time difference; and a functionfor determining a flow velocity dependent time difference from timedifferences determined by said fourth calculating means and said firstcalculating means.